WASHINGTON (AFP) - A US naval ship and surveillance plane are heading to the Andaman Sea and the Bay of Bengal to search for a missing Malaysian airliner that vanished a week ago, officials said Friday.

A P-8 Poseidon aircraft and a guided missile destroyer, the USS Kidd, were due to aid the international hunt for the jet as the search effort extended further west, Pentagon spokesman Colonel Steven Warren said.

"At Malaysia's request, the USS Kidd is north of the Strait of Malacca in what we're calling the western search area," Warren told reporters.

The Kidd was preparing to search the Andaman Sea and the Bay of Bengal for the Malaysia Airlines plane, he said.

"The P-8 will be searching a much larger search area... the southern portion of the Bay of Bengal and the northern portion of the Indian Ocean," he said, adding that final orders had yet to be issued.

The Boeing 777 vanished off radar early Saturday over the South China Sea with 239 people on board, after taking off from Kuala Lumpur bound for Beijing.

The plane's fate has vexed investigators and Malaysia authorities have dramatically expanded the scope of the search operation.

The hunt had initially focused on the South China Sea east of Malaysia - along the intended route of the jet, which took off from Kuala Lumpur en route to Beijing.

But Malaysia's government was now looking at a vast area, extending to the Bay of Bengal and the Indian Ocean, with 13 countries now involved in the desperate search.

The Pentagon said a second naval destroyer, the USS Pinckney, which had assisted the search effort this week, was due to depart for Singapore for scheduled maintenance.

The P-8 aircraft now in the area was replacing a P-3 Orion surveillance plane that had taken part in the initial hunt for the airliner.

After flying over the Gulf of Thailand on Thursday looking for signs of plane debris, the P-3 reported "nothing found," Warren said.